1. Field of the Invention
This invention relates to an impact dot printing head and a method of manufacturing of the same.
2. Description of the Prior Art
Generally, a main section of an impact dot printing head has a construction as shown in FIG. 1. In particular, a spring plate 5 is secured at one end thereof to a frame 1 by way of a spacer 2, a permanent magnet 3 and another spacer 4. Secured to a front face of the opposite free end of the spring plate 5 is an armature 6 made of a magnetic material such as free cutting mild steel and formed as a circular cylinder, and a printing needle 7 which is made of a wear proof material such as cemented carbide and formed as a thin circular cylinder is secured to the armature 6. A yoke 9 is held at one end thereof on the spring plate 5 by way of a spacer 8. The yoke 9 has a hole 9a formed therein into which the armature 6 extends with a little gap left therebetween. A core 10 is secured to the frame 1 and has one end face opposed to the rear face of the free end of the spring plate 5 and the free end of the spring plate 5 is attracted to the end face of the core 10 by magnetic attractive forces. A coil 11 is wound on the core 10 and is electrically connected to an electric circuit (not shown).
Conventionally, the printing needle 7 is secured to the armature 6 in a manner as seen in FIG. 2. In particular, the armature 6 has formed therein a positioning projection 6a in the form of a thin circular cylinder. In assembling, the printing needle 7 is inserted in position into a hole 6b formed at the center of the projection 6a of the armature 6. Then, a piece of silver solder is suitably placed between a front face of the projection 6a and an adjacent portion of a side face of the printing needle 7 of the assembly and is passed through a vacuum furnace to solder the printing needle 7 to the armature 6.
However, according to this invention, the printing needle 7 is soldered to the front face of the armature 6 as described hereinabove, and during passing the assembly through the furnace, melted solder will swell due to its surface tension. As a result, after cooled, the printing needle 7 will vary in diameter at a portion adjacent thereto because of the solder 12, resulting in the necessity of an operation for removing the excess solder. Also, since the solder 12 does not easily go into the hole 6b of the armature 6 and hence does not assure satisfactory soldering result, the solder 12 is easy to remove by a shock when the printing needle 7 is impacted against a platen, that is, when the spring plate 5 is impacted against the core 10 at a high speed, resulting in lack of reliability. Further, the solder 12 must first be formed in prior into a washer or into a ring from a coiled form of stock, and thus it takes a considerable time to shape the solder and to place the shaped solder 12 onto the armature 6, resulting in a rise of production costs of dot printing heads.
Moreover, the printing needle 7 which is required to be wear proof is made of a cemented carbide. Accordingly, because such a cemented carbide will oxidize and become broken if it is heated to or above a melting point of the solder 12 in atmospheric air, it must be soldered within a vacuum furnace as described above in order to prevent such breakage. However, since the armature 6 is made of a free cutting mild steel material, lead gas will generate from such free cutting mild steel if a soldering operation is performed within a vacuum furnace. Such lead gas will have the effect of raising the degree of vacuum while it adheres to an inside surface of the furnace to thereby further cause gas regeneration, leading to the lack of a soldering result of high reliability. In addition, generation of gas as described above causes deterioration of oil in a vacuum furnace.
In this way, such a conventional impact dot printing head as described above is disadvantageous in that it is inferior in quality and reliability and cannot be produced at a low cost.